Denver yesterday gave way to four days of pure political theatre, opening its streets to party enthusiasts in full red, white and blue regalia and anti-war protesters in white death masks, who converged on the city to see Barack Obama declared the Democrats' presidential nominee.

Organisers expect some 50,000 accredited delegates: Democratic governors and members of Congress, party officials and labour leaders. That is a fraction of the attendance at the city's annual livestock show, which drew more than 670,000 ranchers and farmers this year. But Denver authorities hope the convention - seen as the political equivalent of Woodstock - will put the growing city on the map.

The sense of occasion surrounding the convention will culminate when Obama makes his acceptance speech before an audience of 75,000 at Denver's football stadium on Thursday.

There were almost as many police on bicyles, horseback and on foot with assault rifles cradled close to their chests.

The authorities have prepared chain-link holding cells in a disused warehouse in case of arrests - a facility protesters have denounced as a mini-Guantanamo.

Tens of thousands of other activists have descended on the city to make their views known - from disgruntled supporters of Hillary Clinton, leftwing demonstrators wanting to recreate the chaos of the Democratic convention of 1968, opponents of the Iraq war and anti-abortion campaigners.

Amid the protesters will be international observers as well as uninvited guests. The former New York mayor Rudy Giuliani and Mitt Romney, the former Massachusetts governor, who lost their races for the Republican nomination to John McCain, both plan to turn up in Denver during the convention.

Embassies have sent observer contingents, as has the Labour party, which has dispatched a delegation led by the cabinet office minister Ed Miliband, who will have a key role in the next election.

The crush of people, as well as the fact that Obama will be the first African-American to secure the nomination of a major political party, have dictated intense security arrangements overseen by the secret service. About 2,500 FBI and police will be deployed around the convention centre, a secret service spokesman said.

The city's air space will be closed for the duration of festivities. Roads will be blocked off and transit stations shut. The entire Denver police force has been put on standby and the authorities are also pressing prison guards into convention duty.

Delegates are being encouraged to get around on communal bicycles, set up in stands around the city for rent.

Organisers had initially billed Obama's speech accepting the nomination as a public event, promising to give away half the tickets in the stadium to local Coloradans, rather than party grandees. But a cash crunch has compelled the organisers to give away box seats at the stadium to the biggest donors.

Those in prime viewing spots on Thursday night will include the Republican billionaire Tom Golisano, owner of the Buffalo Sabres ice hockey team, who donated $1m to the convention. Big donors are also being invited to attend a post-speech reception where they will meet Obama.